Vinylidene fluoride (VdF) elastomers of various compositions have been suggested. As such VdF elastomers, known are a binary copolymer of VdF and hexafluoropropylene (HFP) (VdF/HFP=60 to 15/40 to 85 (% by weight)) (for example, see JP-B-33-7394); a ternary copolymer comprising tetrafluoroethylene (TFE), VdF and HFP in which TFE is 3 to 35% by weight and the weight ratio of VdF/HFP is 2.33/1 to 0.667/1 (for example, see JP-B-36-3495); a ternary copolymer comprising tetrafluoroethylene (TFE), VdF and HFP in which TFE is 10 to 30% by weight and the weight ratio of VdF/HFP is 1.6/1.0 to 4.0/1.0 (for example, see JP-B-48-18957); and a terpolymer comprising 57 to 61% by weight of VdF, 27 to 31% by weight of HFP and 10 to 14% by weight of TFE (for example, see JP-A-53-149291). Also, as a method for preparing a VdF copolymer, suggested is the method of preparing a VdF elastomer comprising VdF and at least one other fluorinated ethylenic unsaturated monomer containing at least the same number of fluorine atoms as carbon atoms, in the presence of a chain transfer agent (for example, see JP-A-47-5944).
Mold processing of a fluorine-containing elastomer is conducted using common rubber processing machines. That is, mold processing is usually conducted in the order of kneading by a kneading roll or kneader, molding by an extruder, calender roll or press machine, primary vulcanization by press injection and finally, secondary vulcanization by an oven.
However, when only the above fluorine-containing elastomer is used, there were problems regarding mold processing such as roll processability in kneading and mold releasing properties in compression molding are poor, staining of the metal die cannot sufficiently be prevented and flowability in injection molding is poor. In order to solve these problems regarding mold processing, widening the molecular weight distribution was suggested (for example, see JP-A-52-62391 and JP-A-4-209643).
However, by increasing low molecular weight components, compression set resistance decreases and mechanical strength such as tensile strength and elongation and also, solvent resistance tend to become poor. Furthermore, with respect to the objective of obtaining an elastomer having low viscosity to a conventionally unknown degree, it could easily be conceived that the vulcanization properties would decrease further, as low molecular weight components increase.
In this way, properties such as compression set resistance and mechanical properties and mold processability are considered to be opposing properties. In fact, in order to obtain balance of properties and mold processability, the molecular weight and molecular weight distribution of the polymer are adjusted or the structure of the polymer terminals are changed depending on the use or either of the properties are sacrificed.
Furthermore, in a fluorine-containing elastomer composition for sealing material such as O-rings and gaskets, particularly low compression set is required and also, efficient vulcanizability (high vulcanization rate and high crosslinking density) are required. Compression set is an important factor in evaluating sealing performance and when compression set is high, sealing ability is lost in a short period of time. Vulcanizability is a factor that largely influences productivity in mold processing and vulcanizability is preferably as high as possible within the range that required properties are satisfied. Also, in injection molding, excellent flowability is required and when flowability is poor, injecting into the die becomes difficult and more material than necessary must be used. Also, the thickness of the molded article may become uneven.
In order to obtain efficiency in vulcanizability, suggested is the method of decreasing ionic terminals (or acid terminals) derived from ammonium persulfate, which is conventionally used as a polymerization initiator, as much as possible. Examples are the method of using an oil-soluble organic peroxide as the polymerization initiator (for example, see JP-A-6-302487 and JP-A-8-301940) and the method of combining fluoroalkylsulfinate and an organic peroxide (for example, see U.S. Pat. No. 5,256,745). However, these methods are not satisfactory with respect to processability other than vulcanizability.
As a method for simultaneously improving vulcanization efficiency and compression set resistance, the method of adding a phosphonium compound as the vulcanization accelerator is suggested (for example, see JP-A-62-54750). However, even with this method, the problem that processability cannot be improved still remains.
Furthermore, as described above, improvement in flowability has been attempted by lowering the molecular weight, but this is insufficient with respect to compression set resistance and vulcanization efficiency.
In order solve the above problems, a fluorine-containing elastomer, in which the polymer has few branched chains and the weight change in high temperatures is small, and a preparation process thereof have been disclosed (for example, see WO 01/34666). According to this method, a fluorine-containing elastomer that is excellent in vulcanization efficiency, mold processability and compression set of a molded article is obtained, but on the other hand, because the polymer obtained by the disclosed polymerization method has composition distribution, there is the problem that the polymer contains components that differ from the target composition. Furthermore, in order to keep the composition distribution within a specific width, more space than necessary must be acquired and there are problems regarding quality stability and productivity. The reason that composition distribution occurs is because monomers are present in the space in a high concentration and when polymerization is conducted at high pressure, the monomers in the space that is decreased by progression of polymerization are introduced into the produced polymer.
In the conventional art, a polymer of the target composition as the average value is obtained, by decreasing the HFP concentration of the initial monomer in advance, in order to deal with the phenomenon that the HFP concentration in the polymer increases along with progression of polymerization. However, according to this method, because the gaseous phase composition changes largely before and after polymerization, not only does composition distribution of the polymer occur, but also, there is the problem that a polymer of disadvantageous components tends to be produced by post-polymerization after the reaction. Also, because the initial monomers also serve as a buffer for composition distribution in polymerization, a constant amount of the gaseous phase must be acquired and as a result, the amount of monomer blow per batch is large. Thus, productivity is problematic.
On the other hand, the idea behind preparing an elastomer having extremely low viscosity is not disclosed and particularly, as there is no description that a sharp molecular weight distribution Mw/Mn is important, a process for preparing a low viscosity polymer was unknown in reality.
The object of the present invention is to provide a process for preparing a fluorine-containing polymer having few branched chains and little weight change in high temperatures (little thermal decomposition and evaporation), which is a new process wherein composition distribution essentially does not occur.
Another object of the present invention is to provide a fluorine-containing polymer, which has high vulcanization efficiency and excellent mold processability although the viscosity is low to a conventionally unknown degree and gives a vulcanized fluororubber having low compression set, and a composition comprising the same by the above process. Also, the present invention provides a process for preparing vulcanized fluororubber by conducting primary vulcanization of the above composition, while conducting defoaming treatment under reduced pressure.
In addition, the present invention relates to use of the low viscosity polymer as a processing aid.